1. Field of the Invention
The present invention relates to a radio receiver, a radio receiving method, and a recording medium for recording a program to execute the radio receiving method and, more particularly, a radio receiver which has an automatic gain control circuit of a step-wise gain control type in which a gain is changed by a predetermined level when a signal level of a received signal exceeds a predetermined level, a continuous gain control type in which a gain is changed in response to a signal level of a received signal, etc., a radio receiving method, and a recording medium.
2. Description of the Related Art
A radio receiver such as a pager of the day is equipped with an automatic gain control (abbreviated to “AGC” hereinafter) circuit, and then improves the intermodulation desensitization (abbreviated to “IM” hereinafter) characteristic and the characteristic due to an excessive input by controlling a gain of the radio receiver.
Here the IM will be simply explained. The “IM (intermodulation desensitization)” means such a phenomenon that, when a plurality of waves having particular frequency relationship are input into the radio receiver, noises are generated in the receiving band of the radio receiver because of distortion of semiconductor devices such as transistors, diodes, etc.
Also, as the feature of the noise generated by this IM, it is known that the noise level generated in the receiving band of the receiver becomes 1:n when the level of the interference wave having n-th order IM relationship is increased. For example, in the case that two interference waves having frequencies corresponding to the third order IM are input into the receiver, the noise level generated in the receiving band of the receiver is increased by 3 [dB] when the interference wave input is increased by 1 [dB]. In contrast, if the gain of the receiver is attenuated by 1 [dB], the desired wave is attenuated by 1 [dB] but the noise generated by the third order IM is attenuated by 3 [dB]. Therefore, a level ratio of the desired wave to the noise generated by the third order IM can be improved by 2 [dB] rather than the level before the gain is lowered by 1 [dB].
In this manner, in the IM area, desired wave to the interference wave which ensures received signal quality can be improved by controlling (attenuating) the gain of the receiver.
In the prior art, there are two type AGC circuits, i.e., a “continuous gain control type” in which a gain control amount is varied in response to the signal level input into the radio receiver, and a “step-wise gain control type” in which the gain is controlled by a previously determined constant level when the signal level exceeds a predetermined level.
First, the radio receiver having the continuous gain control type AGC circuit in the prior art will be explained with reference to FIG. 19 hereunder. FIG. 19 shows a configuration of the radio receiver having the continuous gain control type AGC circuit (first comparative example) in the prior art.
In FIG. 19, the radio receiver in the first comparative example is constructed to include an antenna 501, a low-noise signal amplifier (LNA) 502, a local oscillator circuit 503, a frequency converter circuit 504, a field intensity detector 505, and a gain control circuit 506.
The antenna 501 receives the signal transmitted from a base station (not shown). The low-noise signal amplifier (LNA) 502 amplifies the signal received via the antenna 501. The frequency converter circuit 504 executes the frequency conversion by multiplying the signal amplified by the low-noise signal amplifier 502 by a signal supplied from the local oscillator circuit 503. The field intensity detector 505 changes a voltage of an output signal GC5 in response to a signal level of the intermediate frequency signal IF after the frequency conversion. The gain control circuit 506 changes a control level of the gain in answer to the output signal GC5 from the field intensity detector 505.
If the level of the signal input into the antenna 501 is changed, the signal level of the intermediate frequency signal IF which is subjected to the frequency conversion by the frequency converter circuit 504 is also changed correspondingly. The voltage of the output signal GC5 of the field intensity detector 505 is changed according to the change in level of the intermediate frequency signal IF, and then a gain control amount of the gain control circuit 506 is changed according to such change.
Next, the radio receiver having the step-wise gain control type AGC circuit in the prior art will be explained with reference to FIG. 21 hereunder. FIG. 21 shows a configuration of the radio receiver having the step-wise gain control type automatic gain control circuit (second comparative example) in the prior art.
In FIG. 21, the radio receiver in the second comparative example is constructed to include an antenna 601, a low-noise signal amplifier (LNA) 602, a local oscillator circuit 603, a frequency converter circuit 604, a field intensity detector 605, and a gain control circuit 606.
Here the antenna 601, the low-noise signal amplifier 602, the local oscillator circuit 603, and the frequency converter circuit 604 have similar functions to those in the first comparative example (FIG. 19). The field intensity detector 605 changes the signal GC6 and outputs it when the signal level of the intermediate frequency signal IF which is subjected to the frequency conversion by the frequency converter circuit 504 exceeds a predetermined value. An operation state (ON state/OFF state) of the gain control circuit 606 is switched by the signal GC6 from the field intensity detector 605. In this case, a gain control amount controlled by the gain control circuit 606 is constant irrespective to the level of the signal input from the antenna 601.
However, in the above radio receiver in the prior art, setting of the radio receiver is decided by searching a point of compromise between the IM characteristic and the electric field variation characteristic.
First, relationships among threshold of electric field intensity level which starts AGC operation, IM characteristics, and margins of sensitivity in the AGC operation in the radio receiver having the continuous gain control type AGC circuit (first comparative example) in the prior art will be explained hereunder. FIG. 20 is a chart showing relationships among threshold of electric field intensity level which starts AGC operation, IM characteristics, and margins of sensitivity in the AGC operation in the continuous gain control type AGC circuit.
As described above, as the feature of the continuous gain control type AGC circuit, it can be listed that the gain control amount is changed pursuant to the change in signal level. However, in the case that the signal level is varied at a pitch that exceeds the follow-up speed of the AGC circuit operation against the change in the signal level, if an electric field intensity variation width of the radio signal exceeds a difference between the sensitivity level and the threshold level of starting the AGC circuit (margin of sensitivity in the AGC operation), the electric field intensity drops down below the sensible level. Therefore, the sufficient margin of sensitivity in the AGC operation must be assured.
Therefore, if the threshold of electric field intensity level which starts the AGC operation is set high, the AGC operation becomes strong against the electric field variation of the desired wave since the large margin of sensitivity in the AGC operation can be assured, but the IM area that can be improved by the AGC circuit can be reduced. Also, in order to improve the IM characteristic, if the threshold of the electric field intensity for starting the AGC operation is set at a low level, the call acceptable area is increased in the IM characteristic, nevertheless the AGC operation becomes weak against the electric field variation of the desired wave since the margin of sensitivity in the AGC operation is small. In this manner, in the continuous gain control type AGC circuit whose gain follows up the electric field variation, the threshold of the electric field intensity level which starts the AGC operation must be set with regard to both the IM characteristic and the electric field variation characteristic.
Then, relationships among the threshold of the electric field intensity level which starts AGC operation, IM characteristics, and margins of sensitivity in the AGC operation in the radio receiver having the step-wise gain control type AGC circuit (second comparative example) in the prior art will be explained hereunder.
In the step-wise gain control type AGC circuit, if the large gain is controlled at a moment, a large noise is generated in the gain control (at the time of switching). Therefore, normally the electric field detection and the gain switching are completed in the signal synchronizing portion, etc., but the gain control amount is not switched in the data interval and thus the receiving operation is carried out while fixing the gain at a constant gain control amount. Therefore, the step-wise gain control type AGC circuit cannot follow up the electric field variation in the data interval. Then, if the electric field intensity variation width of the radio signal exceeds the difference between the threshold of the electric field intensity level which startins AGC operation and the sensitivity in the AGC operation (margin of sensitivity in the AGC operation), the AGC circuit is operated under such a judgment at the time of the electric field detection that the signal level is high strong electric field. In this case, since the electric field intensity drops down below the sensible level with the AGC operation in the data interval, it is impossible to receive the transmitted data. Therefore, the sufficient margin of sensitivity in the AGC operation must also be assured in the step-wise gain control type AGC circuit.
FIG. 22 is a chart showing relationships among threshold of electric field intensity level which starts AGC operation, gain control amounts, and calling rates in the step-wise gain control type AGC circuit. In FIG. 22, the calling rate of 80 [%] is set as the sensible level.
The sensible level in the AGC operation is degraded as reduction in the gain is enhanced by increasing the gain control amount. For example, as shown in FIG. 22, such cases are considered that, if the static sensitivity is 20 [dBuV/m] when the AGC circuit is OFF (at the time of non-operation), the gain control amount is set to 10 [dB] and 15 [dB] respectively. The static sensitivity in the AGC operation is degraded by 10 [dB] and becomes 30 [dBuV/m] if the gain control amount is set small such as 10 [dB] whereas the static sensitivity in the AGC operation is degraded by 15 [dB] and becomes 35 [dBuV/m] if the gain control amount is set large such as 15 [dB]. Also, in the case that the threshold of electric field intensity level which starts AGC operation in the electrostatic field is set to 40 [dBuV/m], the margin of sensitivity in the AGC operation is 10 [dB] if the gain control amount is set to 10 [dB] whereas the margin of sensitivity in the AGC operation is 5 [dB] if the gain control amount is set to 15 [dB]. In this way, in the situation that the threshold of electric field intensity level which starts AGC operation are equal, the margin of sensitivity in the AGC operation is reduced smaller as the AGC gain control amount is set larger.
Then, FIG. 23 is a chart showing relationships among threshold of electric field intensity level which starts AGC operation, gain control amounts, and call acceptable areas in the IM characteristics in the step-wise gain control type AGC circuit.
If it is assumed that the threshold of electric field intensity level which starts AGC operation are equal, the service area existing IM interference is expanded (from AS2 to AS3) if the gain control amount is increased whereas the service area existing IM interference is contracted if the gain control amount is decreased. In this fashion, in the step-wise gain control type AGC circuit, the setting of the AGC circuit must also be set with regard to both the margin of sensitivity in the AGC operation and the IM characteristic.
As described above, regardless of the configuration of the AGC circuit, it is necessary to carry out the setting of the AGC circuit by looking for a point of compromise with regard to both the margin of sensitivity in the AGC operation and the IM characteristic.
Then, it is known that normally the sensitivity of the radio receiver is degraded much more as the transmission data speed of the transmission signal is increased more quickly. Therefore, if transmission conditions such as the transmission data speed of the signal, etc. are different, optimum values of the AGC setting values are different. However, for example, in the radio receiver for receiving the signal having the signal format whose transmission conditions such as the transmission data speed of the transmission signal, etc. are changed into two types or more during the transmission, like the FLEX system pager or FLEX-TD system pager, the same setting of the AGC circuit is applied irrespective to the transmission conditions of the signal. In other words, the optimum setting of the AGC circuit becomes different according to the radio wave situation in which the radio receiver is located. But, a means for executing the setting of the AGC circuit in response to the transmission conditions is not provided to the radio receiver in the prior art and thus it is impossible to set always the AGC circuit optimally.